In normal vision, the two human eyes perceive views of the world from different perspectives due to the separation of the eyes (Interocular separation). These two perspectives are then used by the brain to assess the distance to various objects in a scene. In order to provide a display which effectively displays a three dimensional (3D) image, it is necessary to recreate this situation and supply a so-called stereoscopic pair of images, one to each eye of the observer.
3D displays are classified into two types, namely stereoscopic and autostereoscopic, depending on the method used to supply the different views to the eyes. Stereoscopic displays typically display both of the images over a wide viewing area. In one known type of arrangement, two head mounted separate channels, as in head mounted displays, are worn by an observer such that each channel presents a respective one of the stereoscopic pair of images to the associated eye. Other types of stereoscopic displays typically display both of the images over a wide viewing area. FIG. 1 of the accompanying drawings illustrates such a display at 1 and illustrates the wide output light cone 2 produced by the display. Each of the views is encoded, for instance by colour (in anaglyph systems), polarisation state, or temporally (in shutter glasses systems). Viewing aids such as filters 3 and 4 are worn in front of the right and left eyes R and L of the observer so as to separate the views and let each eye see only the view intended for it. Thus, as shown in FIG. 1, the left and right views are encoded by encodings A and B.
The filter 3, which may be a colour filter, a polarising filter or a shutter, blocks light with the encoding A hut passes light with the encoding B so that the right eye R sees the right view. Similarly, the filter 4 blocks light with the encoding B but passes light with the encoding A so that the left eye sees only the left view.
Autostereoscopic displays require no viewing aids to be worn by the observer. Instead, the two views are only visible from limited regions of space as illustrated in FIG. 2 of the accompanying drawings. The autostereoscopic display 1 creates `viewing regions` such as 6 and 7. The viewing regions are regions of space in which a single two dimensional (2D) image is visible across the whole of the active area of the display 1 by one eye. When an observer is situated such that the right eye R is in the right viewing region 7 and the left eye L is in the left viewing region 6, a stereoscopic pair of images is seen and a 3D image can be perceived.
For flat panel autostereoscopic displays, the viewing regions are typically formed by cooperation between the picture element (pixel) structure of the display and an optical element which is referred to as a parallax optic. Examples of parallax optics are parallax barriers, lenticular screens and holograms. A parallax barrier is a screen with vertical transmissive slits separated by opaque regions. FIG. 3 illustrates an autostereoscopic display of the front parallax barrier type. A parallax barrier 8 is disposed in front of a spatial light modulator 9 comprising glass substrates 10 and columns of pixels 11 with gaps 12 between adjacent columns. The SLM 9 may be a light-emitting device, such as a pixelated electroluminescent display. However, as shown in FIG. 3, the SLM 9 is of the light valve type, such as a liquid crystal device (LCD), provided with a backlight 13.
The pitch of the slits such as 14 is chosen to be close to an integer multiple of the pitch of the columns of pixels 11 so that groups of columns of pixels are associated with each slit of the barrier 8. As shown in FIG. 3, each slit such as 14 is associated with three columns such as columns 1, 2 and 3.
The function of the parallax optic 8 is to restrict the directions in which light is transmitted through each of the pixels to a predetermined range of output angles. To a first order, the angular range of view of each pixel is determined by the pixel width and the separation between the plane of the pixels and the plane of the parallax optic 8.
FIG. 4 of the accompanying drawings illustrates the angular zones of light created by the SLM 9 and the parallax barrier 8 where the parallax barrier has a pitch which is an exact integer multiple of the pitch of the columns of pixels 11. The angular zones Z1 and Z2 coming from different locations across the display surface intermix. Thus, there is no region in front of the display where an eye of the observer will see a single image across the whole of the display surface.
In order to overcome this problem, the pitch of the parallax optic 8 is reduced slightly so that the angular zones Z1 and Z2 converge at a predetermined plane referred to as a `window` plane 15 in front of the display as illustrated in FIG. 5. The change in pitch of the parallax optic 8 is referred to as `viewpoint correction` and gives rise to the viewing regions 6 and 7 where the zones Z2 and Z1, respectively, all overlap. The viewing regions 6 and 7 are generally `kite` shaped in the lateral plane and extend vertically.
The window plane 15 defines the optimum viewing distance of the display. An observer whose eyes are located in the window plane 15 receives the best performance from the display. As each eye of the observer moves laterally in the window plane 15, the perceived image remains unchanged until the eye reaches the edge of the viewing region 6 or 7. The image perceived across the whole display will then change as the eye moves into the adjacent viewing region, for instance to the next image. The part of the window plane 15 within each viewing region 6, 7 is generally referred to as a `viewing window`.
In a typical SLM such as a thin film transistor liquid crystal display (TFT LCD), the columns of pixels 11 are spaced apart by the gaps 12 to allow for the routing of electrical connections. The gaps 12 form vertical strips and are covered by an opaque material to stop light leaking through the gaps. In a TFT LCD, the opaque layer is called a `black mask` or `black matrix`. However, as illustrated in FIG. 6 of the accompanying drawings, the vertical strips between the columns of pixels 11 are also imaged to the window plane and cause dark regions 16 to be formed between the viewing regions 6, 7. If the dark regions 16 are to be avoided so that the viewing regions 6, 7 meet each other, apertures in the black mask defining the pixels 11 must be such that adjacent pairs of pixel columns associated with each parallax element of the parallax optic 8 are horizontally contiguous i.e. there is no continuous vertical black mask strip between such adjacent pairs of pixel columns.
The illumination profile (variation of light intensity with viewing position) within each viewing region 6, 7 is determined by the shape of the apertures defining the pixels 11. The parallax optic 8 is a cylindrical optical element so that the vertical aperture of each pixel column is integrated to give vertically extended lumination within each viewing region 6, 7. Thus, If the vertical aperture of the pixel varies across its width as illustrated at 17 in FIG. 7, illumination of the viewing window varies across its width. This is illustrated in FIG. 7 for the viewing region 7, which is divided into a bright zone 18, a dull zone 19, and mixed zones 20. An observer whose eye moves between the bright zone 18 and the dull zone 19 will perceive an intensity change of the order of 5% or more as a visual flicker effect. This effect detracts from the perceived quality of the display and is uncomfortable. It is therefore desirable to maintain a constant vertical aperture ratio in such displays, for instance by means of rectangular pixel apertures.
If an eye of the observer is not located at the window plane 15, then the breakdown in viewpoint correction means that the eye will see different information in different places across the display surface. For instance, if an observer eye is in the mixed zone 20 closer to the display, the observer eye will see the left hand side of the display as being substantially brighter than the right hand side thereof. If the observer is sufficiently far away from the window plane to be outside the viewing regions 6, 7, each eye sees slices of different images across the display surface so that the 3D effect is lost. This condition begins to occur at the tips of the viewing regions 6, 7 nearest and furthest from the display. Dark bands caused by vertical strips between the pixels also become visible as darker bands on the display.
Although each parallax element is principally associated with a respective group of pixel columns as illustrated by columns 1, 2 and 3 in FIG. 3, adjacent groups of pixel columns are also imaged by the element. Imaging of the groups creates lobes of repeated viewing regions to either side of the central or zero order lobe as illustrated in FIG. 8 for a two view display showing views V1 and V2. Each of the lobes repeats all the properties of the central lobe but is affected to a larger extent by imperfections and abberations of the optical system so that higher order lobes may be unusable.
In order to provide a full colour display, each pixel 11 is generally optically aligned with a filter associated with one of the three primary colours (red, green, blue). By suitably controlling groups of three pixels associated with the three primary colour filters, substantially all visible colours may be produced or approximated. In an autostereoscopic display, each of the stereoscopic image channels must contain enough of the colour filters for a balanced colour output. Many SLMs have colour filters arranged in vertical columns for ease of manufacture so that all the pixels in each column have the same colour filter associated therewith. If a parallax optic were disposed on such an SLM with three pixel columns associated with each parallax element, light imaged into each viewing region would be of only one colour. The colour filter layout must therefore be such as to avoid this situation, for instance as disclosed in EP 0752610.
The autostereoscopic displays illustrated in FIGS. 3 to 7 have parallax barriers as the parallax optics 8 disposed at the front of the display i.e. between the SLM 9 and the viewing regions 6, 7. However, other arrangements of parallax optical work in substantially the same manner.
For instance, as shown in FIG. 9 of the accompanying drawings, the front parallax barrier may be replaced by a front lenticular screen which comprises an array of cylindrically converging lenslets or lenticules. The lenticular screen focuses light from the SLM 9 to the window plane and produces viewing regions having well-defined boundary regions on axis. Because the lenticules work by redirecting light rather than by restricting light throughput as in the case of a parallax barrier, the illumination at the window plane is greater for a lenticular screen. However, parallax barriers are not subject to the optical abberations produced by lenticular screens.
FIG. 10 of the accompanying drawings illustrates an autostereoscopic display which differs from that shown in FIG. 3 in that the parallax barrier 8 is disposed between the backlight 13 and the SLM 9 to form a rear parallax barrier display. This arrangement has the advantage that the parallax barrier 8 is kept behind the SLM 9 and therefore way from possible damage. Also, the light efficiency may be improved by making the rear surface of the parallax barrier 8 reflective so as to permit recycling of the light not incident on the slits (rather than absorbing such light). A switchable diffuser 21 is disposed between the SLM 9 and the parallax barrier 8 and may, for example, comprise a polymer-dispersed liquid crystal. When switched to a low dispersion state, the display operates as described hereinbefore as an autostereoscopic 3D display. When the diffuser 21 is switched to a highly dispersive state, the light rays are deflected on passing through the diffuser to form an even or `Lambertian` distribution which prevents the creating of the viewing zones. The display therefore functions as a 2D display and permits the full spatial resolution of the SLM 9 to be used in displaying a 2D image.
FIG. 11 shows a known type of spatial light modulator (SLM) 9 in the form of a liquid crystal display (LCD) comprising a plurality of picture elements (pixels) arranged as rows and columns in a regular pattern or array. The LCD 9 provides a colour display and comprises red pixels 32, blue pixels 33, and green pixels 34. The LCD 9 is of the thin film transistor twisted nematic type and the pixels are separated from each other by a black mask 35. Thus, each column of pixels is separated from each adjacent column by a continuous vertical opaque strip of the black mask 35, which prevents light from passing through the thin film transistors of the LCD 1.
In order to provide a 3D display, a lenticular screen 8 is disposed in front of the pixels of the LCD 9. The lenticular screen 8 comprises a plurality of vertically extending lenticules, each of which is optically cylindrically converging. The lenticules extend vertically and may be formed, for instance, as plano-convex cylindrical lenses or as graded refractive index (GRIN) cylindrical lenses. Each lenticule is disposed above a plurality of columns of pixels (four columns as shown in FIG. 11) and each column of pixels provides a vertical slice of a 2D view. The shape of each pixel is rectangular with a small rectangular extension projecting from the right side of each pixel.
As illustrated in FIG. 12, when the 3D display is suitably illuminated from behind and image data are supplied to the pixels of the LCD 9 such that each column of pixels displays a thin vertical slice of a 2D image, each lenticule of the screen 8 forms viewing zones 37 to 40 from the respective four columns of pixels associated with the lenticule. The directions in which the viewing zones 37 to 40 extend correspond to the directions from which the respective 2D views were recorded during image capture. When viewed by an observer whose eyes are in adjacent ones of the viewing zones 37 to 40, a 3D image is perceived.
However, the vertical portions of the black mask 35 between the columns of pixels are also imaged in the directions indicated at 41 to 45. Further, the viewing zones 37 to 40 contain regions such as 46 to 48 of reduced brightness corresponding to imaging of the rectangular protrusions extending from the main pixel regions. Thus, the output of the display does not have continuous parallax with uniform brightness.
FIG. 13 shows a 3D display of the type disclosed in EP 0 625 861 and comprising an LCD 9 and a lenticular screen 8. The LCD 9 differs from that shown in FIG. 11 in that the pixels are arranged in a different pattern of horizontal rows and vertical columns. In particular, each pixel may be a composite pixel comprising a red pixel 32, a blue pixel 33 and a green pixel 34. The pixels are arranged such that they are contiguous in the horizontal direction. In other words, there are no continuous vertical black mask portions separating the pixels. To achieve this, each composite pixel 50 in a first row is spaced vertically from a horizontally adjacent composite pixel 51 in a second row but the right hand edge of the composite pixel 50 lies on the same vertical line as the left hand edge of the composite pixel 51. Thus, compared with FIG. 11, the number of columns of pixels imaged by each lenticule of the screen 8 has been doubled to eight whereas the vertical resolution of the LCD 9 has effectively been halved.
As shown in FIG. 14, each lenticule of the screen 8 generates eight viewing zones 52 to 59 which are angularly contiguous with each other and which represent eight different 2D views with continuous horizontal parallax. Thus, "black" regions such as 41 and "grey" regions such as 46 in FIG. 2 are eliminated and an observer can perceive a 3D image of substantially constant intensity and without image gaps. Further, the number of 2D views for the or each 3D image frame is doubled by halving the vertical resolution.
The LCD 9 shown in FIG. 13 thus overcomes the disadvantages of the LCD 9 shown in FIG. 11 in that contiguous viewing zones can be produced. However, the pixels must be accurately contiguous in the horizontal direction in order to avoid undesirable visual artifacts appearing to the observer. In particular, any underlap or overlap of the pixels in the horizontal direction will give rise to intensity variations as an observer eye moves from each viewing zone to an adjacent viewing zone. Thus, LCDs of this type have to be manufactured to very tight tolerances in order to avoid such effects and this increases the complexity and cost of manufacture.
Further, as will be described hereinafter, the crosstalk between left and right views may give rise to undesirable visual artifacts with the LCD of FIG. 13. In particular, the amount of crosstalk seen by each eye may be different and may vary in a stepwise manner as the observer moves.
EP 0 617 549 discloses an stereoscopic head-mounted display which has a separate display device and optical system for each eye of an observer. Each display device comprises a backlight and an LCD and each pedocal system forms a virtual image of a left or right eye view of a stereoscopic pair. For viewing comfort, the virtual images are formed in the same region in front of the observer.
EP 0 262 955 discloses an autostereoscopic display of the type providing two views which are repeated in a plurality of lobes.